1. Field of the Invention
The invention relates to acquire GPS (global positioning system) signals, and in particular, to a GPS receiver and methods for searching and acquiring GPS signals.
2. Description of the Related Art
GPS (global positioning system) technology has been widely used in various devices such as navigation systems of a mobile telephone or a car, based on relative distance between the device and positioning satellites. Each satellite transmits its own CA code through the same RF carrier. The CA code and transmit frequency are known for receiver, but there is a relative speed between the satellite and receiver, the received signal is modulated by Doppler frequency, and there is a space distance between receiver and satellite, the transmit signal need little time to travel the distance and arrive the receiver, which result in a time delay in received signal. In order to obtain the distance and relative speed between the device and certain satellite, the device should recover the CA code of this satellite. If more than four satellites' CA codes are grasped, location and speed of the device can be determined.
If the receiver is static relative to a certain satellite, then the distance can be calculated just by the time delay of the CA code. Time delay can be calculated by measuring the phase shift of the received RA code. Do integrations between the received signal and local CA codes with cyclic shifts, the position of the max integration value represents the phase shift of the received signal.
Nevertheless, there is always a relative movement between the receiver and satellite, and Doppler effect produced, that means the received CA code is modulated by the Doppler and transmitting frequency. If the Doppler frequency is calculated, then the relative speed between receiver and satellite can be calculated.
The GPS receiver should recover the received CA code which is modulated by Doppler and phase shift, the search is two dimensions, one is frequency domain, and the other is time domain.
FIG. 1 shows an example of a search space formed by different code bins and frequency bins. As shown, the space can be referred to as a two dimensional matrix 1, in which each row corresponds to a code bin and each column to a frequency bin. Each cell 10 represents the combination of one code bin 2 and one frequency bin 4, namely, a set of a time delay and a frequency shift. All possible cells 10 are searched by a GPS receiver to locate a target cell through the frequency bin of which the specific code from a specific GPS satellite is received by the GSP receiver, with distance and relative speed between the GPS satellite and the GPS receiver then determinable according to the time delay and frequency shift expressed by the target cell.
In a conventional GPS receiver, the comparison of the correlation value for each cell 10 is performed cell by cell. The search speed is slow. Therefore there exists a need to improve the search speed.